The issue of perchlorate ion contamination of ground water has become a national concern. Perchlorate is a component of certain explosives and has been widely used as an oxidant in solid phase rocket fuels. With each such explosive discharge and each such solid fuel rocket launch amounts of unexpended perchlorate are released into the atmosphere. This perchlorate is extremely water soluble and thus readily enters the ground water. Facilities preparing these explosives and rocket fuels have also been significant sources of perchlorate release. The California Department of Health Services has set an action level for perchlorate ion in drinking water 18 ug/L, based upon the potential for perchlorate to inhibit the uptake of iodine by the thyroid gland.
This perchlorate-contamination problem is a relatively recently identified one and as such a number of competing technologies are under development to solve it.
One approach involves passing the contaminated stream through a bioreactor containing organisms capable of reducing perchlorate to chloride. Another involves the use of ion exchange to remove perchlorate. We have studied the latter approach and have found, as have others, that many ion exchange resins such as common polystyrene/SBA resins have a very high affinity for perchlorate ions. This high affinity, while attractive from the point of view of removing perchlorate from water flows, leads to the problem that it is very difficult to desorb the tightly bound perchlorate from the resin using conventional regeneration techniques such as treatment with concentrated brine. In fact, the amount of sodium chloride needed to regenerate a perchlorate-loaded ion exchange resin can be as much as several hundred pounds per cubic foot of resin. The second regeneration problem is that the spent regeneration brine itself becomes heavily contaminated with perchlorate and the idea of pumping this perchlorate-loaded brine down a disposal well is unattractive in view of the fact that the perchlorate load is likely to eventually reappear in the ground water.
This has led to processes in which the perchlorate ion load in the drinking water stream is adsorbed by ion exchange resin to saturation, the loaded resin is removed and disposed of, such as by burial as hazardous waste or incinerated to destroy its perchlorate content. These disposable resin perchlorate removal processes often function with pairs of beds operated in a “lead-lag” method. In a lead-lag process, the water stream is passed through two resin beds, in series, the first of the two resin beds, the “lead” bed, having a very high affinity for perchlorate, will initially remove virtually all of the perchlorate present in the water stream. The second bed in the series, the “lag” bed will have “nothing to do” during the initial stages of operation as the lead bed is doing such a good job of removing perchlorate. As time passes, the lead bed becomes loaded with perchlorate and gradually begins to loose effectiveness and begins to permit increasing levels of perchlorate to “break through”. Fortunately, in this process, the lag bed is still fresh and it removes any perchlorate passing through the lead bed. This can continue for a period as the lead bed's capacity becomes essentially completely used up. At this point, the lead bed is taken out of service and a new “fresh” bed is introduced , in this case in series with the former “lag” bed which has now moved to a “lead” position.
In a commercial scale water treatment unit, there will typically be two or three “lead-lag” pairs of vessels, all of large (multi-thousand gallon) size and each capable of handling many hundreds of gallons of water flow per minute. This large size leads to the necessity of very substantial permanent installations with heavy foundations and the like. It also leads to the necessity of physically removing the large quantities of hazardous perchlorate-laden disposable resin from the large, permanently mounted vessels at the water treatment site.
We have pioneered the concept of using large numbers of small vessels containing small resin beds in combination with computer control in water treatment settings. We have applied this technology to the removal of nitrate, arsenate and perchlorate from drinking water, as used heretofore, we have operated these facilities under computer control to optimize the removal of ions from water supplies and especially to efficiently regenerate and rinse the resin beds.